Murine or rat crude lymphokine (LK) preparations were produced by Con A stimulation of splenocytes and were partially purified. Addition of these lymphokines to in vitro immunization cultures of murine splenocytes and the antigen sheep erythrocytes (SRBC) resulted in modulation of anti-SRBC antibody production during the in vitro immunization period. Moderate LK concentrations resulted in strong stimulation of antibody production. High LK amounts significantly suppressed the antibody production, but not through a cytotoxic mechanism. Depletion of splenic macrophages from the SRBC in vitro sensitization cultures eliminated normal antibody responses. LK could restore the response to normal levels but, over quite high concentrations, was not suppressive. Reconstitution of cultures with syngeneic, but not allogeneic, plastic-adherent splenic cells restored the ability of LK to suppress antibody production. Human LK preparations could, at moderate concentrations, stimulate the production of anti-tetanus toxoid antibody by human peripheral blood mononuclear cells during in vitro immunization.